Room Paint Coverage Calculator

The basic math

wall area = 2 × (length + width) × height − doors and windows coverage per gallon ≈ 350 sq ft (one coat, smooth surface) gallons needed = (wall area × number of coats) ÷ 350

Always round up. A 12×15 foot bedroom with 9 ft ceilings, one door, two windows, painted in two coats:

• Wall area: 2 × (12+15) × 9 = 486 sq ft
• Minus 1 × 20 (door) + 2 × 15 (windows) = 436 sq ft net
• Two coats: 872 sq ft of coverage needed
• Gallons: 872 ÷ 350 = 2.49
• Add 10% waste: 3 gallons (always round up)

The 350 sq ft rule is optimistic

Paint cans say “covers 400 sq ft” because that’s the lab number. In reality, with cutting in, roller absorption, drying-too-fast in summer, or stippling over textured drywall, you get closer to 300 to 350 sq ft per gallon. Old plaster, raw drywall, and dark-to-light color changes drink paint at 250 sq ft/gallon. Plan conservatively.

The paint coverage calculator uses the same 350, so a room that needs three gallons here needs three there. It used to say 375, which sounds like a rounding difference and is not: on a 12 × 12 room in three coats the two pages landed a whole gallon apart.

When you need more than two coats

• Dark color over white: usually fine in two coats, but reds and yellows often need three (they’re translucent pigments)
• White over dark: almost always three coats. Use a tinted primer first to cut a coat off
• New drywall: PVA primer + 2 coats of paint
• Glossy surface being painted with flat: prime first or paint will streak
• Smoke or water damage: shellac-based primer (Zinsser BIN), then 2 coats

Real coverage by paint type

The dropdown picks the middle of each band: 350 standard, 380 premium, 275 primer. Ceiling paint is handled separately in the result, at 400.

Door and window deductions, kept simple

The standard adjustments most contractors use:

• Door: 20 sq ft (covers a 3’ × 6'8" door + trim allowance)
• Window: 15 sq ft (standard 3’ × 5')
• Sliding glass door: 40 sq ft
• Closet door (bi-fold): 12 sq ft
• Built-in bookcase: roughly its face area

Plenty of painters skip deductions on small openings entirely, on the logic that the leftover becomes touch-up paint. Fine on a small job. Not fine on a long wall with four windows in it, where you would be buying a whole extra gallon for nothing.

Why buying the right amount matters more than you think

Paint stores can re-mix a custom colour, but never identically. Tint formulas drift between batches even at the same store. Run out mid-job, buy another gallon a week later, and the new paint can show as a faintly different shade on the wall, which raking afternoon sunlight will find for you. The only real fix at that point is repainting the whole wall.

The gallon count already carries two margins: 10% waste, then rounding up to a whole can. On the worked example that turns 2.49 gallons into 3, which is most of a spare quart. Add a fourth gallon only when the answer lands just above a whole number, where the rounding gave you almost nothing, or when the colour is a custom mix you could not face re-tinting. Keep whatever is left sealed for touch-ups, where it lasts two to five years.

This page or the coverage one?

This page measures for you. Give it a length, a width, a height and a count of openings, and it works out the wall area, offers the ceiling, and shows how much of the surface the doors and windows take. That is the whole job for one interior room.

The paint coverage calculator starts from a square footage you already have and adds a surface factor, so it is the one for brick, stucco, raw wood, a deck, a fence, or an exterior wall. It also carries stain and masonry coverage rates, which no interior room needs. Measure with a tape, then take the number there.

Sample pots, the smartest few dollars in the whole job

Most paint stores sell 8 oz sample pots. Paint a 2×2 ft swatch on the wall, then look at it morning, noon, and night before committing to 3 gallons of a color. Wall paint looks very different from the chip in store lighting. Most professional painters do this routinely; most homeowners skip it and regret it.

Rough price tiers

Approximate US retail, and they move. Enter what your store actually charges.

A 3-gallon bedroom job at mid-tier paint costs $105 to $165 plus brushes, rollers, drop cloths, painter’s tape, and primer if needed. Budget another $50 to $100 in supplies if you don’t have them already.

The bigger time sink than you expect

Most people underestimate prep by half. Filling holes, sanding, taping, cleaning, moving furniture, taking off outlet covers: that is typically half the job, and the rolling and cutting in goes faster than any of it. Block out a full weekend for one room rather than a Saturday afternoon, and you will finish on Sunday feeling ahead instead of on Monday feeling behind.